Butt Joint Flashing – Why use flashing with Lap Siding?
Why did siding installers start putting pieces of flashing behind each lap siding joint?
In the past, siding installers would caulk the butt joints between pieces of lap siding. This ideally prevented water from seeping behind the siding and running down the water-resistive barrier, where it could pass through nail holes or other weak points and cause rot.
Unfortunately, caulk has a habit of cracking. Over time, expansion and contraction from temperature changes stress the joints. Not only do the butt joints look bad, but their weatherproofing fails too.
With a butt-joint flashing product like JointFlash, that problem disappears. JointFlash sits behind the siding and remains untouched by temperature changes. Made of durable HDPE, it holds up even in extreme weather. And unlike caulk, it doesn’t trap moisture—it allows proper drainage and drying.
JointFlash also installs much faster than caulk. It’s lightweight, so you can carry a full bundle in your nail bag without noticing it. To install, you simply slip the sheet behind the siding piece, nail off the end of that siding, and place your next piece. No extra steps.
If you use James Hardie fiber-cement siding or LP SmartSide lap siding, you’ll want their recommended sizes: Hardie calls for flashing that is at least 6” wide, and LP calls for a flashing that is at least 4″ wide. JointFlash comes in both sizes, saving labor since installers don’t have to cut their own pieces.
JointFlash is also highly cost-effective. Compared to metal and other flashing products, it stands out from other options when comparing both labor and material costs. Metal prices keep rising, while HDPE stays stable.
In short, butt-joint flashing with JointFlash outlasts caulk, speeds up installation, and cuts costs. It’s a small change that makes a big difference.
